Sales/Marketing (which I'll throw Product Managers into that group) should know what the customer is doing/feeling/saying about their products or services. They should understand how other companies are positioning their products and services; and provide that direct feed back to the designers/developers/engineers creating YOUR products and services.
And all those groups should be held accountable for creating what's needed - so it can be delivered to the client at the lowest cost and highest profit possible.
From "FoundRead" Interview with "Coach" Bill Campbell: Engineers should have the ability to say, "This is what we want to do, and all the product managers in the world aren't going to talk us out of this." One day, [a product manager] came to a meeting that included me and banking engineers and says, "I want these features." And I replied, "If you ever tell an engineer what features you want, I'm going to throw you out on the street. You're going to tell the engineers what problem the consumer has. And then the engineers are going to provide you with a way better solution than you'll ever get by telling them to put some dopey feature in there."
Editorial Note: After reading the above (over and over), the essence of the message is right on. Product Managers have not idea how a "feature or function" will affect the design of the product or service. From my background, what I thought was easy, usually turned out to be very, very hard (and the opposite was also true). On the other hand, engineers need to be pushed outside their comfort zone - the majority of them will not go there willingly. So the dance between Product Managers and Product Engineering is more like a very long waltz - where both parties have to share the "lead".